Many chemical reactions are enhanced by irradiation with light of various wavelengths. There are two principal mechanisms by which light may affect chemical reactions. In the type often referred to as photosensitization, a molecule is irradiated with light and attains a relatively long-lived excited state which is then capable of collisionally transfering its energy to a reactant species. The energy transferred to the reactant species causes the latter to change its reactivity, i.e., to chemically react with another reactant species, to dissociate, etc. The role of this type of molecule, conventionally termed a photosensitizer, is primarily to absorb light and transfer the energy generated thereby to a reactant species. A photosensitizer is usually a much stronger absorber of the irradiated light than the species to which it transfers its energy. A photosensitizer permits reactions to occur which otherwise would be difficult or impossible due to the fact that the reactant species absorbs too little light to cause appreciable reaction.
In the other type of reaction, commonly referred to as photoinduced catalysis, a catalytically active species is created by absorption of light. Certain molecules are known to decompose to or become converted to catalytically active species when irradiated with light of certain wavelengths. In this type of reaction, the agent responsible for inducement of the intended reaction is a true catalyst in the process rather than a mere transferor of energy.
It is known (Schmidt et al, J. Chem. Phys., Vol. 51(5), pp. 2024-34 (1969)) that the direct photo excitation of carbonyl sulfide (COS) produces an active catalyst species which will efficiently induce the cis-trans isomerization of olefinic compounds. Schmidt et al theorize that atomic sulfur in its ground electronic state and in an excited electronic state is generated by the photolysis of COS and that the sulfur product of the photolysis acts as the catalytic species for the cis-trans isomerization of olefinic compounds.
A disadvantage in this process, however, is that COS is difficult to directly excite. Thus, COS does not absorb in the near UV wavelengths. The longest wavelength at which COS absorbs is about 270 nm, with maximum absorption occurring at 220 nm. This gives rise to several disadvantages in connection with utilizing this procedure to isomerize olefinic compounds. For example, light sources for this range are generally more expensive and less efficient producers of photons than those which yield longer wavelengths. Moreover, direct excitation of COS is often hindered by olefin absorption of light of shorter wavelengths. Olefin absorption is undesirable in that it wastes energy and may induce unwanted photolysis or side reactions. It would be highly advantageous to extend the wavelength range at which photolysis of COS occurs in order to enable the utilization of less expensive light sources, increase the efficiency of the cis-trans isomerization and to reduce unwanted side reactions.
It is an object of the present invention to provide an improvement in the method for effecting the cis-trans isomerization of olefinic compounds by photo-induced catalysis utilizing COS.